According to the prior art, automatic transmissions, particularly for motor vehicles, comprise planetary gear sets, which are shifted using friction elements or shift elements such as clutches and brakes, and typically are connected to a start-up element, such as a hydrodynamic torque converter or a fluid coupling, which is subject to a slip effect and is provided optionally with a lock-up clutch.
Such an automatic transmission is known, for example, from DE 199 12 480 B4 by the applicant. It comprises three single-carrier planetary gear sets, as well as three brakes and two clutches for shifting six forward gears and one reverse gear, an input shaft and an output shaft, wherein the carrier for the first planetary gear set is continuously connected to the ring gear of the second planetary gear set, and the carrier for the second planetary gear set is continuously connected to the ring gear of the third planetary gear set, and the input shaft is directly connected to the sun gear of the second planetary gear set.
Furthermore, in the known transmission, the input shaft is connectable by means of the first clutch to the sun gear of the first planetary gear set, and by means of the second clutch to the carrier of the first planetary set, wherein the sun gear of the first planetary gear set is connectable by means of the first brake to a housing of the transmission, and the carrier of the first planetary gear set is connectable by means of the second brake to the housing of the transmission, wherein the sun gear of the third planetary gear set is connectable by means of the third brake to the housing of the transmission. The output shaft of the transmission is continuously connected to the carrier for the third planetary gear set and the ring gear of the first planetary gear set.
Furthermore, a nine-speed multi-stage transmission is known from DE 29 36 969 A1; it comprises eight shift elements and four planetary gear sets, wherein one planetary gear set serves as a front-mounted transmission and the main gearing includes a Simpson set and a further planetary gear set serving as reverse gearing.
Further multistage transmissions are known, for example, from the applicant's DE 10 2005 010 210 A1 and DE 10 2006 006 637 A1.
Automatically shiftable vehicle transmissions, of a planetary design, are already generally described numerous times in the prior art and are continually undergoing further development and improvement. These transmissions should have a relatively simple design, in particular requiring a low number of shift elements, and avoid the need for double shifting when sequential shifting is performed, i.e., activation and/or deactivation of two shift elements, thereby ensuring that only one shift element is ever switched when shifting is performed in defined groups of gears.
DE 10 2008 000 428 A1 by the applicant discloses a multi-stage transmission of a planetary design, which includes an input drive and an output drive which are disposed in a housing. With the known transmission, at least four planetary gear sets, hereinafter termed the first, second, third and fourth planetary gear sets, at least eight rotatable shafts, hereinafter termed the drive shaft, output shaft, third, fourth, fifth, sixth, seventh and eighth shafts, as well as at least six shaft elements comprising brakes and clutches are provided, the selective engagement of which generates different transmission ratios between the drive shaft and the output shaft such that preferably nine forward gears and one reverse gear are feasible.
The first and second planetary gear sets, which are preferably designed as minus planetary gear sets, i.e. with a negative stationary transmission, form a shiftable front-mounted gear set, wherein the third and fourth planetary gear sets form a main gear set.
In the known multi-stage transmission, the carriers of the first and second planetary gear sets are coupled together via the fourth shaft, which is connected to an element of the main gear set, the ring gear of the first planetary gear set is coupled to the sun gear of the second planetary gear set via the eighth shaft, which is detachably connectable to the drive shaft via the first clutch, and the sun gear of the first planetary gear set can be coupled to a housing of the transmission by means of the third shaft, via a first brake, and is detachably connectable to the drive shaft via a second clutch, wherein the ring gear of the second planetary gear set can be coupled to a housing of the transmission by means of the fifth shaft via a second brake. In addition, the seventh shaft is permanently connected to at least one element of the main gear set, and can be coupled to the housing of the transmission via a third brake, and the sixth shaft is permanently connected to at least one further element of the main gear set and is detachably connectable to the drive shaft, via a third clutch; the output shaft is permanently connected at least to one further element of the main gear set.
In the known transmission, the fourth shaft is preferably permanently connected to the ring gear of the third planetary gear set, the sixth shaft is permanently connected to the ring gear of the fourth planetary gear set and to the carrier of the third planetary gear set, and is detachably connectable to the drive shaft via the third clutch. Furthermore, the seventh shaft is permanently connected to the sun gears of the third and fourth planetary gear sets, and can be coupled to a housing of the transmission via the third brake. In this case, the output drive is produced via the output shaft, which is permanently connected to the carrier of the fourth planetary gear set. Furthermore, the third and fourth planetary gear sets can be combined or reduced to a Ravigneaux set having a common carrier and a common ring gear.
According to the prior art, the shift elements of a multistage transmission designed in this manner, which are normally designed as multi-disk clutches or brakes, are hydraulically actuated which disadvantageously leads to high hydraulic losses. To circumvent this actuation loss, it would be particularly advantageous to use shift elements which can be actuated as needed.
Shift elements which can be actuated as needed can, in particular, be understood as shift elements that require no, or less, energy to maintain the gear state than to change the gear state, i.e., for example to engage from disengaged, or to disengage from engaged.
To enable the use of shift elements that can be actuated as needed, the shift elements, in particular the clutches, must be easily accessible from the outside.